1. Field of the Invention
The present invention relates to a vehicle position calculating apparatus which is capable of accurately providing the position of a vehicle by employing a combination of an inertial navigation method and a radio navigation method.
2. Description of the Prior Art
An ordinary inertial navigation method employs signals respectively delivered from a traveled-distance sensor and a bearing sensor (a yaw-rate sensor or a terrestrial-magnetic sensor) to carry out a cumulative calculation, thereby obtaining the position of a vehicle.
However, since the inertial navigation method uses a cumulative calculation, possible errors in the signals received from the traveled-distance sensor and the bearing sensor are undesirably accumulated. Accordingly, as the distance traveled by the vehicle increases, the error in calculation of the vehicle position increases cumulatively.
When a terrestrial-magnetic sensor is employed as the bearing sensor, this problem is most particularly aggravated. This is because the direction of the terrestrial magnetism may be offset from the magnetic north by the vehicle's own presence or by surrounding obstructions such as buildings, steel bridges or tunnels, and the signal from the terrestrial-magnetic sensor per se may have a large error, which fact may lead to a remarkably large accumulated error.
On the other hand, when a yaw-rate sensor is employed as the bearing sensor, since only a relative direction is known, it is necessary for the driver of the vehicle to input positional information about two points. This positional information also involves errors which may add to the accumulated error.
Even when the terrestrial-magnetic sensor is employed, it is necessary for the driver to input initial positional information, which fact may cause the accumulated error to be further increased in the same manner as the above.